Preferably is the adverbial form of the adjective preferable. They have the same meaning—better or more desired, even though the adverb preferably is also an alternative way of saying “if possible.” Let’s see the adverb and the adjective in action:

There are a couple of reasons. Preferably comes from the root word prefer and the suffix -able. Preferable is an adjective, and preferably is an adverb. When we add other suffixes to the root word prefer, we double the r.